The present invention relates to a pulse width modulation wave output circuit that uses a ramp voltage to output a pulse width modulation signal.
A power supply device that supplies stable direct current (DC) voltage, which is generated from alternating current (AC) voltage, normally includes a linear regulator or a switching regulator (for example, refer to Japanese Laid-Open Patent Publication No. 5-68372, page 1 and Japanese Laid-Open Patent Publication No. 2006-17462, page 1). A switching regulator is smaller and more efficient that a linear regulator. The switching regulator is one type of a DC stabilizing power supply that is controlled in a switching mode. The switching regulator activates and deactivates a semiconductor switch, such as a power MOSFET, to switch the input power and control the output voltage. The ratio of the ON time and OFF time (duty ratio) of the semiconductor switch is varied to control the output voltage.
In the switching regulator described in Japanese Laid-Open Patent Publication No. 5-68372, to reduce fluctuations of the voltage caused by changes in the input voltage, an input voltage differentiation circuit and a level shift circuit are added to a control circuit of the switching regulator to shift the level of a sawtooth wave in the control circuit in accordance with changes in the input voltage.
In the switching regulator described in Japanese Laid-Open Patent Publication No. 2006-17462, to enable switching operations to be efficiently performed, the output of a triangular wave generation circuit is switched based on a potential at a subsequent stage of an inductor, which smoothes an output potential.
Further, referring to FIGS. 7 to 9, a dual pulse width modulation (PWM) wave output circuit 10, which uses two ramp voltages having the same wave height to output two PWM waves, has been proposed. As shown in FIG. 7, the dual PWM wave output circuit 10 includes two comparators 11 and 12. The comparator 11 receives a reference voltage Vc and a ramp voltage RMP11 from a voltage generator SG1 to output a modulation wave PWM1. The comparator 12 receives the reference voltage Vc and a ramp voltage RMP12 from the voltage generator SG1 to output a modulation wave PWM2.
Referring to FIG. 8A, the ramp voltages RMP11 and RMP12 of the voltage generator SG1 are voltages having sawtooth waveforms. The ramp voltage RMP12 is generated by shifting the minimum voltage level of the ramp voltage RMP11. As a result, as shown in FIG. 8B, the dual PWM wave output circuit 10 outputs the modulation waves PWM1 and PWM2. The overlapping range of the ramp voltages RMP11 and RMP12 may be controlled to control the pulse width of the modulation waves PWM11 and PWM12 with respect to the reference voltage Vc. Further, referring to FIG. 9, the ON/OFF duty ratio of the modulation waves PWM1 and PWM2 may be controlled relative to the reference voltage Vc.
However, Japanese Laid-Open Patent Publication Nos. 5-68372 and 2006-17462 do not consider a buck boosting type structure for overlapped ramp voltage waveforms. When the minimum voltage level of one of the ramp voltages is simply shifted to generate the other ramp voltage, it would be difficult to obtain sufficient amplitude for a low power supply voltage application. Additionally, variations in the ramp voltage wave heights are independent from variations in the voltage shift amount. Thus, variations in the maximum value of the ramp voltage are increased. This limits the dynamic range when the voltage is lowered. Further, the voltage variations increase variations in the overlapped amount. As a result, a PWM wave cannot be accurately output.